Streams of small meteoroid strikes which are almost constantly hitting the moon were found to be infusing the lunar atmosphere with a short-lived water vapour.
When the tiny rock particles hit the moon they penetrate the loose surface matter and the shockwaves liberate water in a “hydrated layer” below the surface.
The findings have provided further evidence to support the hypothesis there are greater and more ancient reserves of water several metres below the surface.
Nasa’s Lunar Atmosphere and Dust Environment Explorer (Ladee) craft, which orbited the moon between October 2013 and April 2014, collected data about the faint gases, known as the “exosphere”, and scientists found when meteoroids hit the surface, in addition to dust, small quantities of water were detected.
“We traced most of these events to known meteoroid streams, but the really surprising part is that we also found evidence of four meteoroid streams that were previously undiscovered,” said Mehdi Benna of Nasa’s Goddard Space Flight Center and the University of Maryland, who is the the lead author of the study, published in Nature Geosciences.
“The water being lost is likely ancient, either dating back to the formation of the moon or deposited early in its history,” he added.
“It’s like if you’re cleaning a big rug by hitting it with a stick,” Dr Benna told New Scientist.
“Every time you hit it, you get a puff of dust over the whole rug because of the shock wave from you hitting one spot with the stick.”
To release water, the meteoroids had to penetrate at least 3 inches (8 centimetres) below the surface, Nasa said.
Underneath the bone-dry top layer there lies a thin transition layer, then a hydrated layer, where water molecules are believed to stick to bits of soil and rock called regolith.
From the measurements of water in the exosphere, the researchers calculated the hydrated layer has a water concentration of about 200 to 500 parts per million, or about 0.02 to 0.05 per cent by weight.
This concentration is much drier than the driest soils on Earth, and is consistent with earlier studies.
It is so dry one would need to process more than a metric ton of regolith in order to collect half a litre of water.
Nasa said despite the moon’s thin exosphere, some of the lofted water does return to the lunar surface, but overall the moon is almost constantly losing water.
“When a stream of meteoroids rains down, the liberated water will enter the exosphere and spread through it. About two-thirds of that vapour escapes into space, but about one-third lands back on the surface,” the agency said.
The research team ruled out the possibility all of the water detected came from the meteoroids themselves.
“We know that some of the water must be coming from the moon, because the mass of water being released is greater than the water mass within the meteoroids coming in,” said the second author of the paper, Dana Hurley of the Johns Hopkins University Applied Physics Laboratory.
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